In Situ Vibrational Spectroscopic Study for Photoelectrochemical CO₂ Reduction over the Au/p-GaN Catalyst: The Role of HCO₃^- for Selective Reaction

The photoelectrochemical (PEC) reduction of CO₂ has been paid much attention to produce fuels and valuable chemicals (e.g., CO, CH₄, CH₃OH). PEC CO₂ reduction is generally performed in solutions with a high concentration of bicarbonate (HCO₃^-) solutions. Recently, gold nanoparticles supported on p-type gallium nitride (Au/p-GaN) exhibited superior selectivity for CO production under visible-light conditions. A determining factor for selectivity could be electrolyte effects. So far, some controversial models have been proposed for the role of HCO₃^- involved in the PEC CO₂ reduction (e.g., proton donor, carbon source). Understanding the role of electrolytes is crucial for controlling the product selectivity. However, only a few studies have addressed how HCO₃^- behaves in PEC CO₂ reduction. In this study, we applied in situ surface-enhanced Raman spectroscopy (in situ SERS) to investigate the behavior of HCO₃^- near the surface during the PEC CO₂ reduction on Au/p-GaN catalysts. The amount of bidentate carbonate (CO₃^2-), which originates from HCO₃^-, takes a maximum at a certain potential (−1.3 V vs Ag/AgCl). Both CO and CH₄ are produced depending on the applied potential as the PEC CO₂ reduction products. The potential dependence of bidentate CO₃^2- can be correlated to the selectivity of CH₄ production. Our results suggest that the HCO₃^- plays a role in improving the selectivity of CH₄ over CO via the adsorption of the CO₃^2- and donation of protons.